In oil fields, drill strings are frequently connected by Hydril-type connectors, which involve a two step thread design, as is well known to those in the art. For instance, Hydril-type connectors are disclosed in U.S. Pat. No. 2,532,632, the disclosure of which is hereby incorporated by reference for the teachings of Hydril-type connectors therein.
The prior art has made extensive use of "O" rings and similar sealing means, in the sealing of pipes and the like. See for instance, U.S. Pat. Nos. 3,054,628; 3,167,333; 2,980,451; 2,889,733; 2,813,567; 2,770,477; and 2,553,340. Generally the sealing rings or gaskets of these prior art patents have been made of Teflon or similar plastic material.
The use of rings of plastic or rubber or the like in rotary tools is also known to the art, see, for instance, U.S. Pat. Nos. 2,102,072 and 2,110,127. Finally, the use of Teflon as a sealing ring in Hydril-type connectors is disclosed in U.S. Pat. Nos. 2,907,589 and 3,100,656. In these patents, the Teflon seal ring is located in the area of the free terminal forward end of the pin member of the Hydril-type connector. If such sealing rings are so located on the pin member, they are highly susceptible to damage, and if located on the box member they are quite difficult to install, and subject to a tendency to be locally deranged in an unpredictable manner as the joint is made up.
Many different designs of screw threaded pin and box type tubing and drill pipe joints have been proposed by the prior art. However, in spite of the many proposals, a number of problems have remained unsolved, especially those problems having to do with a method of adequately and enduringly sealing a Hydril-type connector after the metal-to-metal seating surfaces have been damaged, which is a frequent occurrence, against the extremely high fluid pressures which are encountered in deep drilled oil and gas wells. For instance, such pressures might range as high as 15,000 psi.
The rubber O-rings and compressible rubber gaskets, such as those acknowledged hereinabove, have from time to time been used to seal pipe and tubing joints, and under certain operating conditions, especially when relatively low pressures are involved, they have proven to be thoroughly satisfactory. However, such seals have not been suitable for use under the high temperature and pressure conditions which are encountered in deep oil and gas wells. For one thing, these relatively thin rubber rings tend to be badly damaged, and often rendered useless, by being extruded under the well pressure into small clearances between the joint members at the seal ring groove. Compressive ring gaskets, such as of rubber and asbestos, have also been found to be susceptible to damage, probably as a result of a combination of sliding friction and high compressive loads, during make up of the pipe joint with power tools.
Joints used for the purpose of connecting drill pipe lengths or tubing lengths must be made up with a tight fit on tapered lateral contact surfaces in order to provide the required degree of rigidity needed to resist lateral bending stress with a minimum of fatigue. It is, however, difficult to produce sealed joints having both a tight fit on tapered, lateral contact surfaces, and a longitudinally compressed ring gasket, as even very small variations in the machining of the tapered lateral conduct surfaces results in significant displacement variations in the longitudinal make up of the joints, with resultant differences of degree in the amount of compression applied to the ring gasket, which will sometimes be damaged by too great a compression, or may be compressed insufficiently to adequately seal the joint.
Difficulties of this nature have led to the use of special pipe and tubing joints which have tapered sealing elements employing highly polished metal-to-metal fits as a means of sealing against very high oil pressure, and such joints, when made up carefully under ideal conditions, are entirely capable of sealing against such high well pressures. This type of joint, referred to hereinafter as the Hydril-type connector or joint, is generally more costly to manufacture than more conventional pipe and tubing joints, and the polished sealing surfaces or seats are quite susceptible to damage when the joints are made up and broken out under operating conditions encountered in the oil fields. Damage may occur as a result of direct or sliding impact of the pin seal against the end of the box when the lower end of a suspended, relatively heavy pipe stand is stabbed into a joint on the upper end of the pipe which precedes the suspended pipe stand into the well.
Furthermore, the polished metal-to-metal sealing areas or seats are also quite susceptible to damage at the time the joints are screwed together, as even quite small particles of dirt or sand, if trapped between the mating seats, are capable of scoring or producing galling which may completely destroy the effectiveness of the metal-to-metal seal. Excessive friction, caused by overtightening, lack of lubrication, or rapid make up, will occasionally cause the tight fitting metal-to-metal seats to gall during the last fraction of a turn as the joints are screwed together.
In an effort to avoid the entrapment of dirt and sand particles between the seating surfaces as the joints are screwed together, the prior art has customarily made the metal-to-metal contact area between the seats relatively narrow. The prior art may provide one joint member with an arcuate seating surface and the mating member with a conical seating surface to obtain a line of contact between the two surface. However, such narrow seating surfaces are not generally desirable because they are quite vulnerable to attack by the corrosive fluids which are often present in oil and gas wells. In this regard, the corrosive fluids need penetrate only a relatively short distance in order to completely destroy the line of metal-to-metal contact existing between the two joint surfaces, and thereby destroy the seal.
Thread damage is frequently caused by the vertical stabbing of threaded pin ends of suspended heavy pipe stands into threaded box ends of pipe or pipe collars supported beneath the pipe stands. This damage usually occus when the entire stabbing load falls on the relatively thin and frail thread ends of the threaded pin and box members. A damage end thread on one joint member will often damage, and occasionally destroy, a number of additional threads on both members should the pin and box be screwed together before the thread end damage is discovered.
The Hydril-type connectors, because of their relatively delicate sealing areas, are easily damaged. Damage to the Hydril-type connector sealing areas has heretofore required that the unit be scrapped, or shop repaired at great expense.
The Hydril Company commercially offers a patented tubing connection, with a Teflon ring located in the box member at a location such that when the connection is made up the ring will be at the free terminal end of the pin. It is indicated that the prime purpose of this ring is to complete the corrosion protection afforded by plastic coated pipe. The advertising material also indicates that the connection may be specified where an additional high-pressure seal is desired.
The Rucker Atlas Bradford Company has offered leak-proof tubing to the trade, using conventional thread profile, but modifying the coupling by machining out one and one-half threads near each end of the internally threaded coupling and installing a Teflon ring in the resultant retaining groups. In another version, a pin and box member design is provided wherein a groove is machined inside the box member to retain a Teflon seal ring.
U.S. Pat. No. 4,085,951 to James B. N. Morris, issued Apr. 25, 1978, discloses the use of a secondary sealing means between the stepped threaded areas of a Hydril-type tubing or casing connector or joint. The secondary seal is an annular ring of relatively rigid internally tenacious plastic or rubber material, such as polytetrafluoroethylene, which serves to contain well pressure if the metal-to-metal sealing surfaces on the pin and/or box members of the Hydril-type connector are damaged. With the sealing rings disclosed in the Morris patent, damaged Hydril-type connectors can be used without having to undergo extensive repair of the damaged area, or even scrapping.
FIGS. 5, 6, 7 and 8 of the Morris patent disclose various forms of sealing rings which can be used therein, with various modifications of the inner wall of the annular ring being made to facilitate installation of the ring over a portion of the threads of the Hydril-type connector. Column 5, lines 3-9 of the Morris patent also suggest that a ring of square cross-sectional area, as illustrated in FIG. 8, may be installed by heating the ring until the ring expands enough to permit it to be slipped over the pin member threads, or alternatively suggest the ring could be actually molded in place in the ring-receiving groove.
The sealing ring of the Morris patent has been found to work very effectively, especially when used to contain liquids. However, the sealing ability of the sealing ring of the Morris patent is pressure limited by the contact pressures which can be applied to the ring, and it has been found that high pressure gases can leak relatively easily past the relatively small sealing area of the sealing ring of the Morris patent.